1. Technical Field
The present disclosure relates to a howling removing apparatus and a howling removing method each of which prevents the howling with use of an adaptive filter.
2. Description of the Related Art
In an environment where a microphone and a speaker are located at positions close to each other, a sound signal picked up by the microphone is amplified and output from the speaker, and the output sound signal thus amplified is transferred through a space and is picked up again by the microphone, whereby an acoustic closed loop is formed. As a result, the sound signal output from the speaker and input to the microphone again is amplified repeatedly, whereby howling occurs.
There is known a technology, in order to prevent the howling that occurs as described above, for calculating an adaptive filter by estimating spatial transfer characteristics in a course where the sound signal, which is output from the speaker, is transferred through the space and is input to the microphone with use of an adaptive algorithm. In accordance with this technology, the obtained adaptive filter and the sound signal output from the speaker are convoluted with each other, whereby a signal that simulates a speaker output transferred through the space is obtained. This simulation signal is subtracted from the sound signal picked up by the microphone, whereby it becomes possible to remove only the speaker output, which is transferred through the space and is input to the microphone to cause the howling, and as a result, such an occurrence of the howling can be prevented.
In the adaptive algorithm that updates the adaptive filter with use of an output of a system, which is changed from moment to moment, the adaptive filter sometimes oscillates depending on a state of the output. When the adaptive filter oscillates, a failed abnormal signal (for example, a noise signal always having maximum amplitude) is output irrespective of a magnitude of an input signal. Moreover, since there is always some value in the adaptive filter, an output signal does not become 0, and the abnormality signal continues to be output even if the input signal stops.
In order to prevent such oscillation of the adaptive filter, a fail-safe function is generally implemented in a system using the adaptive algorithm. For example, Japanese Patent No. 4428247 discloses; a method of calculating a sound pressure level of an input signal and a sound pressure level of an output signal, and initializing an adaptive filter when a difference therebetween exceeds a threshold value, thereby restoring an output to normal; and a method of initializing an adaptive filter when a difference between such an un-updated adaptive filter and such an updated adaptive filter, both of which represent a correction amount of the adaptive filter, exceeds a threshold value, thereby restoring an output to normal.